The present invention relates to a sealing device for explosion-proof motors. More particularly, the invention relates to a sealing device for preventing any flames generated within the casing of the motor from escaping outside through the gap between the driving shaft and the casing of said motor.
It is known that explosion-proof motors, i.e., motors in which the safety aspect is essential because they must operate in dangerous environmental conditions, for example in the presence of flammable or explosive substances, have a sealing device which is arranged between the shaft of the motor and the casing and is rigidly coupled to said casing.
Explosion-proof motors can be manufactured, depending on the load to be driven, with driving shaft supports equipped with rolling bearings or with a plain bearing (bushing).
In the latter case, the coupling tolerances between the shaft and the support are greater than in the rolling-bearing solution. Given an equal precision class, machining tolerances are a function of size and increase as said size increases.
Accordingly, larger driving shafts require tolerances which are higher in terms of absolute value and are more critical as regards providing the seal.
Qualification tests for motors are performed while the motor is motionless and/or rotating.
In view of what has been described above, the qualification of bushing-supported motors is more critical for the following reasons:
the size of the passage through the sealing device is larger;
when the motor is not moving, since the shaft does not have the hydrodynamic support generated by the film formed by the oil when the motor turns, said shaft rests on the bushing. Since the sealing device is rigidly coupled to the casing of the motor, in this situation the thickness of the passage in the sealing device appears to be twice bigger than that when the motor is running and when the shaft is supported by means of a rolling bearing.
Accordingly, the considerable thickness of the passage makes the motor unsuitable for use as an explosion-proof motor.
Moreover, the driving shaft must be shaped appropriately so as to form a sort of labyrinth which loosely couples in the complementarily shaped sealing device.
FIG. 1 is a schematic view of a conventional driving shaft 1, whose outer surface is machined so as to form a labyrinth-like profile 2 which accommodates the sealing device 3 having a complementarily shaped labyrinth-like profile and being rigidly coupled to the casing 4 of the motor.
In order to allow the driving shaft 1 to rotate freely, a certain clearance, designated by the reference numerals 5 and 6 in FIG. 1, must necessarily be left.
Since the driving shaft 1 must rotate within the sealing device 3, it is also necessary to leave, between the shaft 1 and the sealing device 3, a certain space which does not comply with standards concerning explosion-proof motors.